Akihiro Sasoh

Review: Laser-Ablation Propulsion

LASER ablation propulsion (LAP) is a major new electric propulsion concept with a 35-year history. In LAP, an intense laser beam [pulsed or continuous wave (CW)] strikes a condensedmatter surface (solid or liquid) and produces a jet of vapor or plasma. Just as in a chemical rocket, thrust is produced by the resulting reaction force on the surface. Spacecraft and other objects can be propelled in this way. In some circumstances, there are advantages for this technique compared with other chemical and electric propulsion schemes. It is difficult to make a performance metric for LAP, because only a few of its applications are beyond the research phase and because it can be applied in widely different circumstances that would require entirely different metrics. These applications range from milliwatt-average-power satellite attitude-correction thrusters through kilowatt-average-power systems for reentering near-Earth space debris and megawatt-to-gigawatt systems for direct launch to lowEarth orbit (LEO). We assume an electric laser rather than a gas-dynamic or chemical laser driving the ablation, to emphasize the performance as an electric thruster. How is it possible for moderate laser electrical efficiency to givevery high electrical efficiency? Because laser energy can be used to drive an exothermic reaction in the target material controlled by the laser input, and electrical efficiency only measures the ratio of exhaust power to electrical power. This distinction may seem artificial, but electrical efficiency is a key parameter for space applications, in which electrical power is at a premium. The laser system involved in LAP may be remote from the propelled object (on another spacecraft or planet-based), for example, in laser-induced space-debris reentry or payload launch to low planetary orbit. In other applications (e.g., the laser–plasma microthruster that we will describe), a lightweight laser is part of the propulsion engine onboard the spacecraft.

Laser-driven in-tube accelerator

A device of accelerating a projectile using laser power, “laser-driven in-tube accelerator,” has been developed. It is characterized by accelerating the projectile in a tube and by supplying the laser beam from the direction opposite to the projectile motion. The laser beam is incident on the nose of the projectile, reflected first on the nose and second on the inner wall of the projectile shroud, then focused behind. Operation performance is measured using a CO2 transversely excited atmospheric laser of a nominal output energy of 5 J/pulse. A coupling coefficient of 73 N/MW is obtained using the atmospheric air as the working fluid.

Numerical Simulation of Galileo Probe Entry Flowfield with Radiation and Ablation

A trajectory-based heating analysis of the Galileo probe entry flowfield is attempted to reproduce the heat-shield recession data obtained during the entry flight. In the calculation, the mass conservation equations for the freestream gas (hydrogen-helium gas mixture) and the ablation product gas are solved with an assumption of thermochemical equilibrium. The ablation process is assumed to be quasi steady and is coupled with the flowfield calculation. The radiative energy transfer calculation is tightly coupled with the flowfield calculation, where the absorption coefficients of the gas mixture are given by the multiband radiation model having 4781 wavelength points for wavelength range from 750 to 15,000 A. The injection-induced turbulence model proposed by Park is employed to account for the enhanced turbulence effect due to the ablation product gas

Electromagnetic effects in an applied-field magnetoplasmadynamic thruster

Experimental and analytical studies have been conducted on the performance and thrust production mechanisms of an applied-field magnetoplasmadynamic thruster. The thruster was able to run with a high-thruster performance due to large electromagnetic effects related to the applied magnetic field. Using hydrogen, helium, and argon as the propellant, over 20 percent thrust efficiency was obtained over a wide specific impulse range from 1000 to 7000 s at input power levels between 2.2 and 15.9 kW. From the measurements of performance characteristics and current densities in the acceleration region, and by a theoretical analysis, it is found that the thruster operation is characterized by a parameter, B-squared/m (B: applied magnetic field strength, m: propellant mass flow rate). 9 refs.

Supersonic Drag Reduction with Repetitive Laser Pulses through a Blunt Body

A drag over a flat-nosed cylinder with repetitive laser pulse irradiations ahead of it were experimentally measured in a Mach-1.92, in-draft wind tunnel. Laser pulses were focused using a plane-convex lens fabricated on the nose of the cylinder at a repetition frequency of up to 10 kHz and power of 70 W at a maximum. The drag was measured using a low-friction piston which was backed by a load cell in a cavity at a controlled pressure. Under the experimentally available operation conditions, an up-to-3 % drag reduction and an efficiency of energy deposition of about 10 were obtained. Mechanisms of the drag reduction are analyzed based on experimental flow visualization and numerical simulation.

A high‐resolution thrust stand for ground tests of low‐thrust space propulsion devices

A new type of thrust stand for accurately measuring a low‐level (10 mN to 1 N) thrust produced by a space propulsion device in its ground tests has been developed. Vacuum bellows are utilized both as a feedthrough tool from electrical power, propellant, and coolant supplies to a thruster installed in a vacuum chamber, and as an elastic support. In order to improve thrust resolution, friction force on radial bearings at a fulcrum is minimized by eliminating bearing radial forces. 0.5 mN thrust resolution was experimentally obtained. Utilizing thrust stand vibration characteristics measured in advance, a computer‐processed, quick thrust reading system has been established.

Ambient Pressure Dependence of Laser-Induced Impulse onto Polyacetal

L ASER ablation can be usefully employed to generate a propulsive impulse on an object not only in the atmosphere but also in vacuum [1–3]. Larson et al. [4] proposed a launch vehicle propelled by repetitively pulsed laser ablation. Several authors [5–9] show that favorable propulsion performance in the atmosphere can be achieved with a polymer material, polyacetal, which is commercially named “Delrin” or is abbreviated as “POM.” The ablated gas from this material does not contain much air pollutant. Targeting space applications, the laser-ablative-propulsion performance of metals and polymers at low ambient pressures has been intensively investigated [10–17]. For many kinds of metals and polymers, Cm ranges from 10 to 100 N-s=J, and depends strongly on the characteristics of the laser pulse. According to measurements byGregg andThomas [10], metallic materials have an optimum laser intensity that maximizes Cm. Phipps et al. [12] formulated experimental Cm characteristics of aluminum alloys and several polymers in terms of the intensity, width, andwavelength of the laser pulse.Cm can be further increased by utilizing the so-called “volume absorber” [13] or layered target [14]. Recent measurements by D’Souza and Ketsdever [17] of Cm for polyacetal at low ambient pressure using a frequency-doubled Nd:YAG pulsed laser showed peak of 110 N-s=J at a laser intensity of the order of 10 W=cm. Several authors investigated the influence of ambient pressure on Cm [18,19]. Pakhomov et al. [18]measured the impulse generated on an aluminum surface irradiated by a CO2 laser pulse whose pulse width was 200 ns; Cm decreased monotonically with decreasing ambient pressure. In Dufresne et al.’s surface-pressure measurement [19], the ambient-pressure dependence of Cm for aluminum was quite sensitive to the laser pulse width. For polymer materials, Beverly and Walters [20] measured the CO2-laser-induced shock pressure in cellulose acetate and polymethylmethacrylate. The peak shock pressures exhibited the complex dependence on the ambient pressure. The ablative impulse dependence of materials other than aluminum warrants further investigations. To quantitatively evaluate the feasibility of laser-ablativepropulsion systems, the propulsive performance of polyacetal over a wide range of ambient pressure needs to be known. In the present paper, the influence of the ambient pressure on the impulse characteristics of polyacetal is investigated experimentally using a transversely excited atmospheric (TEA) CO2 pulse laser.

A weak shock wave reflection over wedges

When a weak shock wave reflects from wedges its reflection pattern does not appear to be a simple Mach reflection. This reflection pattern is known to be von Neumann Mach reflection in which a Mach stem can not necessarily be straight. In this paper the local change of the Mach stem curvature was experimentally and numerically investigated. A distinct triple point, at which the curvature becomes infinite as appears in a simple Mach reflection, was not observed but the Mach stem curvature became a maximum between foot of the Mach stem and a point, P1, at which an incident shock met with a reflected shock. Maximum curvature point P2 and P1 do not coincide for small wedge angles and tend to merge over a certain wedge angle. Experimental results agreed with numerical results. The trajectory angle of P2 was found to be expressed well by Whithams shock-shock angle.

Thrust Formula for Applied-Field Magnetoplasmadynamic Thrusters Derived from Energy Conservation Equation

A thrust formula for applied-field magnetoplasmadynamic (MPD) thrusters has been derived from the energy conservation equation for the propellant plasma flow by assuming that the work done by electromagnetic forces is converted into kinetic energy of the exhaust plasma. Three acceleration mechanisms, 1) generalized Hall acceleration, 2) swirl acceleration, and 3) self-magnetic acceleration are taken into account. This formula enables theoretical calculation of thrust of an applied-field MPD thruster from controllable thruster operation parameters, and estimation of the contributions of the respective acceleration mechanisms.

SIMPLE FORMULATION OF MAGNETOPLASMADYNAMIC ACCELERATION

A simple formulation of magnetoplasmadynamic acceleration has been made based on energy conservation relations and a generalized Ohm’s law. An exhaust velocity is expressed using three characteristic parameters: (1) a dimensionless characteristic velocity U≡μ0J2D/(muA) (JD is the discharge current, m is the mass flow rate of working fluid, uA is the Alfven’s critical velocity, and μ0 is the permeability in vacuum); (2) the ratio of an applied to self‐induced magnetic fields; and (3) an electron Hall parameter. An exhaust velocity calculated using the formula agrees well with the experimentally measured value.